In your room, place the wanted object in the very upper left corner of your room.
Let's say that the sprite is 32x32.
In the Create_Event of obj_block:
var obj_block;
if instance_count < 100
{
instance_create(x+32,y,var);
instance_create(x,y+32,var);
instance_create(x+32,y+32,var);
}
In the Step Event of obj_block:
if place_meeting(x,y,obj_block) {instance_destroy();}
I think this will work, I didn't try it out before I wrote it just now, but I've tried it before.